Considering the potential interference of ultra-wide hand (UWB) system to OFDM based narrowband wireless communication system, such as IEEF. 802.1 In system, this paper investigates the optimal power allocation scheme for Impulse Radio UWB (1R-IIWB) user in order to maximize the total transmission rate of UWB with the constraints that its interference to IEEE 802.11n user is below a certain threshold. Therefore, UWB user could reach its best performance regarding to transmission rate, while the coexisting IEEE 802.11n user tends to work normally with permissible interference from UWB user. Simulation results show that the optimal power allocation scheme significantly increases the capacity of UWB user compared with equal power allocation scheme.